At the self service checkout, you have the option to enter a barcode number manually rather than scanning. The last 4 digits of the barcode on the yellow discount sticker are the price, so enter 0001 instead.

I would think probably in the evening when they start putting the stickers on stuff. Any later then they might just throw it all out....
Also not to mention if you're entering barcodes manually overnight they might think you're trying to blag the scales on what items you have so best off doing it when they're more distracted with other people.